OXIDATIVE ADDITION OF SILANES TO CYCLOPENTADIENYLBIS(PHOSPHINE)CARBONYLMANGANESE - FLUXIONAL BEHAVIOR OF MANGANESE SILYL HYDRIDE COMPLEXES

Photolysis of hydrocarbon solutions of CpMn(CO)(P2) (P2 = DMPE,DMPP,DMPM, (PMe3)2, respectively, 1, 3, 5, 7) in the presence of SiPhnH4-n yields the corresponding new manganese silyl hydrides CpMn(P2)(H)(SiPhnH3-n) (2a-c, 4a-c, 6,8) in good to excellent yield. Complex 2b crystallizes in space group P2(1)/n (No. 14) and its structure was refined to a final R factor of 7.41 %. The molecular structure of 2b is a distorted three-legged piano-stool geometry with an apparent vacant coordination site presumably occupied by the hydride ligand. All of these complexes undergo net exchange of the relative hydrido and silyl positions with DELTAG double dagger(ex) almost-equal-to 9.6-11.9 kcal/mol. The magnitude of DELTAG double dagger(ex) appears to depend upon the size (or lack) of the chelate ring. A deuterium labeling experiment shows that neither intermolecular exchange nor intramolecular ''windshield wiper'' exchange of the bound vs unbound Si-H bonds is observed in these systems. These results are most consistent with a pseudorotation of a seven coordinate Mn center resulting in mutual exchange of the silyl and hydride sites. The mechanism of formation of 2, 4, 6, and 8 appears to involve initial phosphine dissociation followed by silane addition. Loss of CO occurs from the formally Mn(III) center followed by ring closure to give the product.